EP0790708B1 - Architecture de conversion numérique-analogique améliorant l'élimination de signaux parasites - Google Patents

Architecture de conversion numérique-analogique améliorant l'élimination de signaux parasites Download PDF

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Publication number
EP0790708B1
EP0790708B1 EP97101555A EP97101555A EP0790708B1 EP 0790708 B1 EP0790708 B1 EP 0790708B1 EP 97101555 A EP97101555 A EP 97101555A EP 97101555 A EP97101555 A EP 97101555A EP 0790708 B1 EP0790708 B1 EP 0790708B1
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European Patent Office
Prior art keywords
output
stage
dac
interpolation
rate
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Expired - Lifetime
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EP97101555A
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German (de)
English (en)
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EP0790708A2 (fr
EP0790708A3 (fr
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Brent A. Myers
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Harris Corp
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Harris Corp
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/66—Digital/analogue converters
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/06—Continuously compensating for, or preventing, undesired influence of physical parameters
    • H03M1/08—Continuously compensating for, or preventing, undesired influence of physical parameters of noise
    • H03M1/0863—Continuously compensating for, or preventing, undesired influence of physical parameters of noise of switching transients, e.g. glitches
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/66—Digital/analogue converters
    • H03M1/68—Digital/analogue converters with conversions of different sensitivity, i.e. one conversion relating to the more significant digital bits and another conversion to the less significant bits
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/66—Digital/analogue converters
    • H03M1/74—Simultaneous conversion
    • H03M1/80—Simultaneous conversion using weighted impedances
    • H03M1/802—Simultaneous conversion using weighted impedances using capacitors, e.g. neuron-mos transistors, charge coupled devices
    • H03M1/804—Simultaneous conversion using weighted impedances using capacitors, e.g. neuron-mos transistors, charge coupled devices with charge redistribution

Definitions

  • the present invention relates to digital to analog conversion method and an interpolating digital to analog converter with improved spurious energy rejection.
  • Systems for modulating digital data desirably suppress spurious energy in order to facilitate modulation to a higher frequency within a finite transmission bandwidth.
  • an input data stream is conditioned (or shaped) with a pre-modulation baseband processor 10 in Figure 1.
  • Conventional digital interpolation techniques e . g ., finite impulse response (FIR) filter 12
  • FIR finite impulse response
  • a digital to analog converter (DAC) 14 running at the interpolation rate approximates the pulse shape in discrete time form.
  • Conditioning of the pulse shape is completed in a continuous time reconstruction filter 16 where the discrete time nature of the pulse is smoothed to suppress spurious energy which occurs at the update rate of the DAC 14.
  • the reconstruction filter 16 should be of a high order (typically an order greater than four) for high performance systems. It is also desirable that such filters can be manufactured in a monolithic integrated circuit. However, high order reconstruction filters are very difficult to manufacture in a monolithic integrated circuit (if it can be done at all), and an alternative approach is desired.
  • the approach proposed herein is to reduce the need for a high order reconstruction filter by improving the spurious energy rejection in the DAC which precedes it. For example, if the spurious energy occurring at the DAC output rate is reduced by one order of magnitude compared to the prior art converter, the order of the reconstruction filter may be reduced from at least four to one.
  • the present invention DAC is based on charge redistribution, and use a cascaded design to reduce the need for precisely matched capacitors and resistors (see, for example, "High-resolution switched-capacitor D/A converter" by R. Gregorian, Microelectronic Journal , Vol. 12, No. 2, 1981.)
  • the cascaded design does not afford the desired reduction of spurious energy and further improvements are needed.
  • the present invention includes a switched-capacitor interpolation filter as an integral component of the DAC.
  • the filter output is fed back to a conversion stage of the DAC to smooth the analog output and thereby reduce the spurious energy in the output. While switched-capacitor interpolation filters are known, there has been no suggestion to integrate them into a DAC.
  • the present invention includes a digital to analog conversion circuit as defined in claim 1 and a method of reducing spurious energy in an analog signal which is output from a digital to analog converter (DAC) as defined in claim 6.
  • DAC digital to analog converter
  • the present invention recognizes that digital-to-analog conversion is a discrete time process, and increases the output rate over the input rate by interpolating the amplitude values.
  • the interpolation herein allows the invention to reduce the spurious energy content of the output signal by an order of magnitude to thereby permit use of a less complex reconstruction filter.
  • the process may be described as a two step charge redistribution with feedback to interpolate between samples. DC offset is minimized by using double sampling techniques which permit a fully held signal between interpolation samples.
  • Figure 2 shows an embodiment of the interpolating digital-to-analog converter 20 of the present invention may include a first conversion stage 22 for converting the first n bits ( e . g ., the n least significant bits) of an N bit data signal received at an input rate to a first output value in line 24, a second conversion stage 26 for combining the remainder of the N bits ( e .
  • Figures 3 - 5 illustrate first conversion stage 22 that may include an offset compensated amplifier connected to a stable reference.
  • First stage 22 includes a first capacitor array 40 (for which see Figure 4) with n parallel paths 42 with capacitors C therein which are switched into and out of operation by switches d1, d2,...dn which are respectively responsive to the first n data bits b1, b2, ..., bn gated with clock ⁇ 1.
  • Figure 5 illustrates the use of AND gates to provide signals a1, a2, ..., an for operation of switches d1, d2,...dn.
  • the prefix to C indicates a multiple of the unit capacitance C in a preferred embodiment, although other values may be used.
  • Switches 44 discharge capacitors C during one of the time periods ⁇ 1 and ⁇ 2 which alternate at the sampling frequency (the time period shown next to a switch indicates a preferred time period, although the indicated time periods may be swapped in another embodiment.)
  • the sampling frequency is greater than the digital data update frequency
  • the interpolation factor is the ratio of the sampling frequency to the data update frequency. For example, an update frequency of 1.024 MHz and a sampling frequency of 5.12 MHz yields an interpolation factor of 5.
  • the output of capacitor array 40 is provided as an input to a non-inverting terminal (-) of a first operational amplifier 46.
  • a switched-capacitor circuit 48 may be provided for amplifier 46.
  • the output of amplifier 46 may be provided as the first output value in line 24.
  • Second conversion stage 26 includes a second capacitor array 50 with (N-n) parallel paths, each with a capacitor therein, which are switched responsive to the remainder of the N data bits.
  • Second capacitor array 50 may be identical tc first array 40 shown in Figure 4, except for the data bits to which the switches d1,...dn respond ( i . e ., bn + 1, bn + 2,...bN gated by clock ⁇ 2) and the time period in which switches 44 are closed (during ⁇ 1 instead of ⁇ 2.)
  • capacitor array 50 The output of capacitor array 50 is connected to node 51 which is a summing node for the first stage least significant bit conversion network ( i . e ., for bits 1 to n) through capacitor E in line 24, and capacitor array 50, which is the conversion network for the most significant bits ( i . e ., n + 1 to N).
  • a switched-capacitor circuit 54 may be provided to hold the charge transferred from the summing operation.
  • the resulting output of amplifier 52 may be provided in output line 28 prior to interpolation stage 30.
  • Interpolation stage 30 includes a third operational amplifier 56 with a switched-capacitor circuit 58.
  • Second stage 26 and interpolation stage 30 are desirably integrated into a single component in a monolithic integrated circuit DAC. The combination of the two stages reduces the number of components and reduces the necessary silicon area.
  • Integral stages 26 and 30 also includes a feedback circuit 34 with a feedback capacitor F which introduces a complex pole so that DAC stage 26 and interpolation stage 30 together act as a second order filter 60. Hence, design of the desired interpolation response is reduced to the design of a simple second order filter. The resulting interpolated output is available at node 32.
  • Connections between the stages may include switched-capacitors to ensure continuity therebetween, such as switched-capacitor E in line 24 between first stage 22 and second stage 26, and switched-capacitor D in line 28 between second stage 26 and interpolation stage 30.
  • the embodiment of Figure 3 builds on the circuitry of the prior art DAC.
  • the second stage of the prior art DAC is replaced in the present invention by filter network 60 (stages 26 and 30 and feedback circuit 34) which improves rejection of spurious signals.
  • the first conversion stage is typically followed by an identical second conversion stage which generates the most significant bits portion of the analog signal.
  • the first stage would sum into a second stage with an attenuation of 16 to realize the least significant bits portion of the signal.
  • the embodiment of Figure 3 replaces the conventional second stage with a filter designed to eliminate offset effects while realizing a second order transfer function.
  • This filter network, 60 is incorporated directly into the DAC and is clocked at a higher speed (five times the rate at which the digital word is being updated in one embodiment) thus smoothing the DAC output.
  • the offset correction of the second conversion stage 26 effectively removes dc offset at the output from operational amplifier 56.
  • the voltage offset of operational amplifier 52 may be set to zero by the correlated double sampling technique illustrated in Figure 3.
  • the offset of operational amplifier 52 is sampled during one phase ( e . g ., ⁇ 2) and removed during the other phase ( ⁇ 1).
  • the resulting output has a second order transfer function with the offset removed.
  • FIG. 6 An example of the five times interpolation is illustrated in Figure 6 in which curve A is an output from a DAC without the integrated interpolating filter of the present invention and curve B is an output from a DAC incorporating an embodiment of the present invention.
  • the DAC update rate was 1.024MHz and the interpolation rate was 5.12MHz.
  • the interpolating DAC of this example provides no more than 0.2dB loss at 100KHz while giving 20dB extra attenuation at the spurs of 1.024MHz. This gives nearly 37dB of spurious rejection, 20dB over that achievable in the prior art.
  • the present invention may provide any reasonable interpolation factor by varying capacitor values in consideration of the clock frequencies. Selection of appropriate capacitances of capacitors A, B, D and F set the second order filter transfer function and hence the interpolation factor so as to reduce the spurious energy in the output by an order of magnitude compared to the spurious energy in outputs from simple prior art converters, thereby permitting a single order reconstruction filter following the DAC. As is apparent, a somewhat similar result may be achieved by adding a filter to the cascaded DAC of the Gregorian article. In contrast, the present invention integrates the filter into the DAC and adds capacitors A, B, D, and F and operational amplifier 56. The mere addition of a filter to the Gregorian circuit would require these additions and substantially more. Thus, the present invention improves produceability and spurious energy rejection while using less silicon area than would be required without the present invention.
  • a digital-to-analog conversion method and interpolating digital-to-analog converter for a data modulation system which reduces the spurious energy content of the output signal by an order of magnitude to thereby permit use of a less complex reconstruction filter to smooth the analog output.
  • the process is a two step charge redistribution with feedback to interpolate between samples. DC offset is minimized by using double sampling techniques which permit a fully held signal between interpolation samples.
  • a first conversion stage converts the first n bits of an N bit data signal received at an input rate to a first output value, and a second conversion stage converts the remainder of the N bits and combines signals from the two conversion stages to provide a combined output to an interpolation stage which provides an interpolated output at an interpolation output rate.
  • a feedback circuit provides the interpolated output to an input of the second conversion stage.
  • a multistage charge redistribution digital to analog converter comprising a filter network which has a network data conversion stage integral with an interpolation stage for improving suppression of spurious signals in an input signal to the DAC, said interpolation stage being clocked at a rate which is a multiple of the input rate of the DAC input signal, and for feeding back an output therefrom to said network data conversion stage to smooth an analog signal output from the DAC.
  • a digital-to-analog conversion method and interpolating digital-to-analog converter for a data modulation system which reduces the spurious energy content of the output signal by an order of magnitude to thereby permit use of a less complex reconstruction filter to smooth the analog output.
  • the process is a two step charge redistribution with feedback to interpolate between samples. DC offset is minimized by using double sampling techniques which permit a fully held signal between interpolation samples.
  • a first conversion stage converts the first n bits of an N bit data signal received at an input rate to a first output value, and a second conversion stage converts the remainder of the N bits and combines signals from the two conversion stages to provide a combined output to an interpolation stage which provides an interpolated output at an interpolation output rate.
  • a feedback circuit provides the interpolated output to an input of the second conversion stage.

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  • Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Analogue/Digital Conversion (AREA)

Claims (7)

  1. Circuit de conversion numérique-analogique comprenant un convertisseur D/A fonctionnant à une fréquence d'interpolation pour convertir un signal de données à N binaires reçu à une fréquence d'entrée en un signal analogique dans lequel l'énergie parasite du signal de données reçu est réduite dans le signal analogique, le convertisseur D/A étant d'un type à redistribution de charge, caractérisé en ce que le convertisseur DAC comprend un circuit de filtre qui possède un étage de conversion de données de circuit (26) intégré à un étage d'interpolation, ledit circuit de filtre (60) améliorant l'élimination de signaux parasites dans un signal d'entrée dans le DAC, ledit étage d'interpolation (30) étant rythmé à une fréquence qui est un multiple de la fréquence d'entrée du signal d'entrée du DAC, et ledit circuit de filtre (60) renvoyant à partir de lui une sortie vers ledit étage de conversion de données du circuit (26) pour lisser une sortie de signal analogique du DAC.
  2. Convertisseur DAC selon la revendication 1 comprenant, de plus, un premier étage antérieur de conversion de données pour recevoir le signal d'entrée dans le DAC et pour convertir n binaires d'un signal d'entrée de N binaires dans le DAC en une première valeur de sortie qui est fournie audit étage de conversion de données du circuit (26), ledit étage de conversion de données de circuit (26) servant à convertir les binaires restants du signal d'entrée à N binaires en fine seconde valeur de sortie et à combiner les première et seconde valeurs de sortie afin de fournir une valeur de sortie combinée, et ledit étage d'interpolation (30) comprend un filtre de commutation de condensateur (40) pour recevoir la valeur de sortie combinée et fournir une sortie interpolée à une fréquence d'interpolation qui représente ledit multiple de la fréquence d'entrée.
  3. Convertisseur numérique-analogique (DAC) selon la revendication 1, caractérisé en ce qu'il comprend:
    un premier étage de conversion (22) pour convertir n binaires d'un signal de données à N binaires reçu à ladite fréquence d'entrée en une première valeur de sortie;
    le circuit de filtre (60) comprenant, de plus :
    un second étage de conversion (26) pour convertir le reste des N binaires du signal de données en une seconde valeur de sortie, et pour combiner les première et seconde valeurs de sortie en une valeur de sortie combinée,
    l'étage d'interpolation (30) pour recevoir la valeur de sortie combinée venant dudit second étage de conversion (26) et fournir une sortie interpolée à une fréquence de sortie d'interpolation qui est un multiple de la fréquence d'entrée, et
    un circuit de retour (34) pour renvoyer la sortie interpolée venant dudit étage d'interpolation (30) vers ledit second étage de conversion (26),
    de sorte que la sortie interpolée est le signal analogique présentant une énergie parasite réduite.
  4. Convertisseur DAC selon la revendication 3 dans lequel ledit circuit de retour (34) comprend un condensateur (F) qui en liaison avec ledit second étage de conversion (26) forme un pôle complexe pour fournir une sortie interpolée.
  5. Convertisseur DAC selon l'une quelconque des revendications 1 ou 3, dans lequel ledit étage d'interpolation (30) comprend un premier amplificateur opérationnel (56) et des premier et second condensateurs commutés (A, B) dans des chemins parallèles connectant une entrée et une sortie dudit premier amplificateur opérationnel (56), et ledit étage d'interpolation (30) est connecté audit second étage de conversion (26) à travers un troisième condensateur commuté (F).
  6. Procédé de réduction d'énergie parasite dans un signal analogique qui est délivré par un convertisseur numérique-analogique (DAC) qui convertit un signal de données à N binaires reçu à une fréquence d'entrée en un signal analogique, le procédé comprenant les étapes consistant à :
    a) convertir dans un premier étage de conversion (22) n binaires d'un signal de données à N binaires reçu à une fréquence d'entrée en une première valeur de sortie;
    b) combiner dans un second étage de conversion (26) le reste des N binaires du signal de données avec la première valeur de sortie afin de fournir une sortie de valeur d'amplitude ;
    c) fournir une sortie d'amplitude interpolée venant d'un étage d'interpolation (30) qui interpole la sortie de valeur d'amplitude à une fréquence de sortie d'interpolation qui est un multiple de la fréquence d'entrée; et
    d) renvoyer la sortie d'amplitude interpolée vers une entrée dans le second étage de conversion (26) ; de sorte que l'énergie parasite de la sortie d'amplitude interpolée est une forme réduite qui est obtenue par une conversion directe du signal de données.
  7. Procédé selon la revendication 6 dans lequel l'étape de renvoi comprend l'étape consistant à former un pôle complexe.
EP97101555A 1996-02-14 1997-01-31 Architecture de conversion numérique-analogique améliorant l'élimination de signaux parasites Expired - Lifetime EP0790708B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US08/601,401 US5798724A (en) 1996-02-14 1996-02-14 Interpolating digital to analog converter architecture for improved spurious signal suppression
US601401 1996-02-14

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EP0790708A2 EP0790708A2 (fr) 1997-08-20
EP0790708A3 EP0790708A3 (fr) 1999-07-28
EP0790708B1 true EP0790708B1 (fr) 2001-08-08

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US (1) US5798724A (fr)
EP (1) EP0790708B1 (fr)
KR (1) KR970063948A (fr)
CN (1) CN1168570A (fr)
DE (1) DE69705976T2 (fr)
TW (1) TW341009B (fr)

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Publication number Publication date
EP0790708A2 (fr) 1997-08-20
DE69705976T2 (de) 2002-04-04
KR970063948A (ko) 1997-09-12
DE69705976D1 (de) 2001-09-13
CN1168570A (zh) 1997-12-24
EP0790708A3 (fr) 1999-07-28
TW341009B (en) 1998-09-21
US5798724A (en) 1998-08-25

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